1. Field of the Invention
The present invention relates to an optical fiber light coupling interface with an enlarged incident surface to couple the light power from a light source to an optical fiber, and especially to a light coupling device of an optical fiber which inputs the light power dispersed from such a light source i.e., a laser diode, or a light emitting diode, to an enlarged incident end-face which emits diverging light, and outputs it to an optical fiber.
2. Prior Art
A number of conventional light coupling devices, each of which couples light power from a light source to a single-mode optical fiber, have been proposed and used. A conventional light coupling device is constructed using a lens system consisting of one or more optical lenses, and the system is arranged in a space between a light source and an optical fiber end-face. In this type of light coupling device, the spot radius of the light beam radiated from the light source is adjusted to be the mode radius of the optical fiber core in order to improve the efficiency of the light coupling between the light source and the optical fiber.
A reflection light loss at a surface of an optical lens in an optical conventional coupling device is approximately 14% for each lens. Optical power (Pa) incident on a core of an optical fiber can propagate effectively through the optical fiber core of the conventional light coupling device.
Since a core of a single mode optical fiber is 9 to 10 .mu.m in diameter, existing alignment errors of the optical axes among the light source, optical fiber, and optical lens system remarkably decrease the efficiency of light coupling.
Efficiency .eta. of light coupling is given in terms of the alignment error among the axes of these optical components by expression (1): EQU .eta.=exp(-2d.sup.2 /.omega..sup.2) (1)
where d is an alignment error(.mu.m), and .omega. is the mode radius of the optical fiber. Assume that efficiency .eta. of light coupling at the alignment error of 0 .mu.m is 100%. Efficiency .eta. of light coupling for d=2.5 .mu.m and .omega.=5 .mu.m is then given as approximately 60%.
If an optical lens system is used to propagate a light beam in an optical coupling device, it is necessary alignment errors of the optical axes among the optical components to be corrected. Efficiency .eta. of the light coupling system depends on the accuracy of correction. In addition, reflection light losses at the surfaces of the lenses are added each time the light beam passes through a lens surface. Due to these difficulties, efficiency .eta. of the light coupling of the system was limited to at most approximately 40% in most cases.
A light coupling device of direct coupling type or a light coupling device of simple structure, wherein no lens system is arranged in the space between the light source, i.e., a laser diode or a light emitting diode, and the single-mode optical fiber end-face, as shown in FIG. 4, has been recommended.
FIG. 4 shows the principle of operation of a light coupling between a light source and an optical fiber end-face according to the proir art. Here no lens system is arranged in the space between the light source and the optical fiber end-face. In FIG. 4, 1 denotes a light source, i.e., a laser diode, 2 denotes the core of an optical fiber 6, 3 denotes the clad of the optical fiber 6, 4 denotes the total light beam radiated, and the light beam 5 incident on the core 2 of the optical fiber 6. When a laser diode is used as the light source, the intensity of light 4 emitted from the laser diode 1 in radiation angle .theta.r is distributed in accordance with the Gaussian distribution, and the light beam is coherent. Due to the diffraction of coherent light beam with the Gaussian distribution, an elliptical radiation pattern is formed. The elliptical pattern has a spread of 40 to 60 degrees along the XX' axis and a spreading of 20 degrees along the YY' axis. Light power Pa incident on the core 2 of the optical fiber is calculated by: EQU Pa=I.sub.0 {1-exp(-2a.sup.2 /.omega..sub.z.sup.2)}
where I.sub.0 is the intensity of the light power emitted from the light source, "a" is the radius of the core of the single-mode optical fiber (=5 .mu.m), and ".omega..sub.z " is the radius of the light beam incident on the optical fiber end-face, at a distance z measured from the light source. The average radiant angle .theta..sub.r of the total light flux 4 is assumed to be 25 degrees. The numerical aperture for the light power incident on the core of the single-mode optical fiber is assumed to be NA=.theta..sub.1 =5.3 degrees.
Light power Pa incident on the end-face of optical fiber core 2 is calculated to be approximately 8%. Remaining light power Pa, which is approximately 92% of the total light power, is incident on optical fiber clad 3 and other areas. The light power incident on optical fiber clad 3 is radiated to outer surface 6 of the optical fiber clad 3 and results in becomes a radiation loss.
Assume that optical fiber end-face 7 approaches light source 1 of a laser diode as much as possible; in that case, the light power incident on the optical fiber core 2 at the incident angle of NA=.theta..sub.1 =5.3 degree or more cannot propagate along the optical fiber, although the light power incident on optical fiber core 2 at an incident angle of less than NA=.theta..sub.1 =5.3 degrees can propagate along the optical fiber.
A light coupling device of simple structure, which is built in accordance with the direct coupling structure as shown in FIG. 4, is easy to build, but impractical in most cases due to its low efficiency of light coupling. However, since no optical lens is used to simplify the configuration of the assembly in the above method of direct coupling, a number of variations have been proposed to improve the efficiency of light coupling.
For instance, a light coupling device shown in FIG. 5 represent such a variation according to the prior art, and is described in "Ideal Microlenses for Laser to Fiber Coupling" by Christopher A. Edwards, et.al., IEEE Journal of Lightwave Technology, Vol. 11, No. 2, PP. 252-257, (February 1993).
FIG. 5 shows an example of a cross-sectional view of the light coupling device constructed in accordance with the method of direct coupling.
A tapered portion 9 wherein the radius of an optical fiber 8 is reduced toward the end-face of the optical fiber 8 is formed by fusing and drawing the optical fiber 8 so that the mode radius of a core 10 is extended, and a hemisphere microlens 11 is formed at the top of the tapered portion 9 due to surface tension caused by fusing the optical fiber end-face.
In FIG. 5, distance z between a light source of laser diode 1 and the optical fiber end-face is 8.5 .mu.m, and radius R of the surface curvature of the microlens 11 is 5.7 .mu.m. The efficiency .eta. of the light coupling is reported to be approximately 50%. In this example, the numerical aperture is small because of the very small radius of microlens 11. Errors can occur in aligning the optical axes between the light source and the optical fiber, which both limit the efficiency of light coupling of the device.
An optical connector employing field modification invented by Nolam, et.al., is disclosed in U.S. Pat. No. 4,763,976, will be explained referring to FIG. 6. FIG. 6 shows a cross-sectional view of the optical connector according to the prior art. In FIG. 6, a glass tube 15 having optical refractive index n.sub.3, which is smaller than the optical refractive index n.sub.1 of an optical fiber clad 14, is concentrically arranged in a unit structure around outer surface 13 of the clad 14 of an optical fiber 12, and the end-face of the optical fiber 12 is finished to be small by fusing and drawing the various elements together. In the example shown in FIG. 6, the mode radius of optical fiber core 16 is extended to be twice as large as the normal mode radius for the normal optical fiber so that the efficiency of light coupling might not be decreased even if an alignment error has occurred in between the optical axes. The proposed device shown in FIG. 6 is aimed to improve the efficiency of light coupling.
When the ratio of the end-face diameter to the normal optical fiber diameter is 1 to 4 in the tapered portion, the mode radius .omega. of the optical fiber core is reported to be 10 .mu.m. If the offset (d) of the optical axes between the light source and the optical fiber is 2.5 .mu.m, the efficiency (.eta.) of light coupling is calculated to be 88% because only the offset of the optical axes is considered to decrease the efficiency of light coupling.
From the technical point of view, finishing of an optical fiber to make a tapered portion in the shape shown in FIG. 6 is however not so easy, and the practical fabrication is considered to be difficult.
Both alignment errors due to the offset of the optical axes between the light source and the optical fiber, and the numerical aperture (NA) of the optical fiber core reduce the light power which is effectively input to the optical fiber core. These limiting factors reduce the efficiency of light coupling to 50% or less in many conventional devices.